To a suspended mixture of 7-bromo-3-(6-hydroxy-1,3-benzodioxol-5-yl)-3-(hydroxymethyl)-1,3-dihydro-2H-indol-2-one (4.70 g, 12.4 mmol) in anhydrous tetrahydrofuran (120 mL) was added tributylphosphine (3.8 mL, 15.5 mmol), followed by additional of di-tert-butyl azodicarboxylate (3.58 g, 15.5 mmol) in anhydrous tetrahydrofuran (25 mL) at 0° C. The reaction mixture was stirred at 0° C. for 1 h and at ambient temperature for 16 h. The reaction was quenched by addition of saturated aqueous ammonium chloride solution (150 mL) and extracted with ethyl acetate (3×200 mL). The combined organic solution was washed with brine (100 mL), dried over anhydrous sodium sulfate, filtered and concentrated in vacuo to dryness. The residue was purified by column chromatography with ethyl acetate in hexanes (10% to 30% gradient) to give 7′-bromospiro[furo[2,3-f][1,3]benzodioxole-7,3′-indol]-2′(1′H)-one (2.4 g, 53%) as a colorless solid: mp 240° C. (dec.); 1H NMR (300 MHz, DMSO-d6) δ10.88 (s, 1H), 7.34 (dd, J=8.1, 1.0 Hz, 1H), 7.10 (d, J=7.2 Hz, 1H), 6.93 (dd, J=8.1, 7.5 Hz, 1H), 6.68 (s, 1H), 6.36 (s, 1H), 5.93 (dd, J=2.9, 0.8 Hz, 2H), 4.77 (d, J=9.4 Hz, 1H), 4.65 (d, J=9.4 Hz, 1H); 13C NMR (75 MHz, DMSO-d6) δ178.1, 155.3, 148.2, 141.6, 141.1, 134.2, 131.4, 123.8, 122.8, 119.4, 103.1, 102.0, 101.3, 93.1, 79.9, 58.8; MS (ES+) m/z 360.2 (M+1), 362.2 (M+1); MS (ES−) m/z 358.2 (M−1), 360.2 (M−1).
